In X-ray image acquisition technology, an object to be examined, e.g. a patient, is situated between an X-ray generating device or X-ray source, e.g. an X-ray tube, and an X-ray detector. A fan-beam or cone-beam is generated by the X-ray source, possibly employing collimation elements, in the direction of the X-ray detector. The object to be examined situated in the path of the X-radiation is spatially attenuating the X-ray beam, depending on its inner structure. The spatially attenuated X-radiation is subsequently arriving at the X-ray detector, with the intensity distribution of the X-radiation being determined and subsequently converted to electrical signals for further processing and display of an X-ray image.
Both the X-ray generating device and the X-ray detector may be mounted on a gantry for rotation about the object to be examined. By providing an according rotation with subsequent acquisition of different X-ray images of varying alignment and orientation with respect to the object to be examined, a three-dimensional reconstruction of the objects inner morphology may be obtained.
However, a certain object may have only a minor attenuation of X-radiation or differences in attenuation even within different tissues in the inside of the object, thus resulting in a rather uniformly attenuated X-ray image having low contrast and so impeding distinguishing individual elements of the interior of the object to be examined. While different regions within the object may have similar attenuation properties, they may influence a phase of X-radiation penetrating the object to a larger extent.
Thus, phase-contrast imaging may be employed for visualization of phase information of X-radiation, in particular, at least partly, coherent X-rays, passing an object to be examined. In addition to X-ray transmission imaging taking into account only amplitude attenuation of X-radiation, phase-contrast imaging may not only determine absorption properties of an object to be examined along a projection line, but also the phase-shift of transmitted X-rays. A detected phase-shift may thus provide additional information that may be employed for contrast enhancement, determining a material composition, possibly resulting in a reduction in X-radiation dosage.
Since a phase of a wave may not be measured directly, a conversion of a phase-shift into an intensity modulation by interference of two or more waves may be employed.
In differential phase contrast imaging, the use of a cone-beam geometry may constitute a limitation of the usable size of an X-ray detector element, in particular when the phase and/or the absorption gratings are aligned with their trenches parallel to the optical axis. At a distance of about lm from the x-ray source, the point where the phase-sensitivity drops significantly with respect to the central region of the imaging system is about +−3 cm off the optical axis. This limitation may in particular depend on grating properties, visibility, distance and angle of a cone beam or fan beam.
For some applications, e.g. medical imaging applications, inspection imaging applications or security imaging applications, a field of view of fewer than 6 cm, at least in one direction of a two-dimensional X-ray image may be too small to be feasibly reasonable. Moreover, for phase contrast imaging, multiple images of a region have to be acquired, having individual phase stepping states for a preferred reconstruction of image information.
Thus, there may be a desire to increase the field of view of an image obtainable when employing phase-contrast imaging while reducing acquisition steps necessary due to phase stepping while acquiring image information.